A printing apparatus used as a printer, copying machine, or the like, or a printing apparatus used as an output device for a multifunction electronic apparatus or workstation including a computer and wordprocessor prints an image (including characters, symbols, and the like) on a print medium such as a paper sheet or thin plastic plate on the basis of printing information. Such printing apparatuses are classified by the printing method into an inkjet type, wire dot type, thermal type, laser beam type, and the like.
In a so-called serial type printing apparatus which performs printing operation while scanning a printhead in a direction (main scanning direction) perpendicular to the print medium conveyance direction (sub-scanning direction), a print medium is conveyed by a predetermined amount every time an image is formed by printing operation of scanning the printhead once along the print medium. After conveyance of the print medium is stopped, the print medium undergoes printing operation by the next scanning. By repeating this processing, printing is done on the entire print medium.
To the contrary, in a printing apparatus which incorporates a full-line printhead for only conveying a print medium in printing operation, a print medium is set at a predetermined position and conveyed while printing operation of every line is continuously performed, thereby printing on the entire print medium.
Among printing apparatuses of the above-mentioned types, a printing apparatus which adopts an inkjet printing method (inkjet printing apparatus) prints by discharging ink from a printhead onto a print medium. The inkjet printing apparatus has many advantages: the printhead can be easily downsized, a high-resolution image can be formed at a high speed, the running cost is low because printing can be done on a plain sheet without requiring any special processing, noise is hardly generated because of non-impact printing, and an arrangement to form a color image with multicolor ink can be easily employed.
Of these printing apparatuses, a line printer using a so-called full-line printhead configured by arraying many printing elements and many ink orifice nozzles in a direction perpendicular to the print medium conveyance direction can achieve a higher image formation speed, and attracts attention as a candidate for a printing apparatus which satisfies recent needs for on-demand printing
On-demand printing is required to be laborsaving, instead of realizing high-speed printing of, e.g., 100,000 sheets per hour, unlike conventional printing of newspapers and magazines in millions of copies. The line printer using a full-line printhead is much lower in printing speed than a conventional printing apparatus for offset printing type or the like, but can save labor and is suitable for on-demand printing because no printing plate need be formed.
The line printer which adopts the full-line printhead and is used for on-demand printing must print on A3-size print media by 30 pages or more per minute at a resolution of 600×600 dpi (dots per inch) for a monochrome printing document such as a text and a resolution of 1,200×1,200 dpi or more for a full-color image such as a photograph. Also, the line printer must flexibly print on print media of a plurality of sizes: for example, the printer outputs an image photographed by a digital camera or the like on an L-size print medium, similar to a conventional printing apparatus, or the printer prints on a small print medium such as a postcard.
It is, however, difficult to faultlessly machine and manufacture all ink discharge nozzles (to be referred to as nozzles hereinafter) and printing elements arrayed along the entire width of the printing area in the line printer using the full-line printhead. For example, a full-line printhead used for a line printer which prints a photographic output on a large-size paper sheet, like business documents in an office, requires about 14,000 nozzles (printing width of about 280 mm) in order to print at a resolution of 1,200 dpi on an A3-size paper sheet. It is hard for the manufacturing process to faultlessly machine and manufacture such a large number of nozzles and corresponding printing elements. Even if a printhead can be manufactured faultlessly, the fraction defective and manufacturing cost rise.
To prevent this problem, there is proposed a line printer type inkjet printing apparatus using a full-line printhead which is implemented by a so-called “connecting head”. The “connecting head” is a printhead which attains a long printing width with arraying at high precision a plurality of printhead chips which are adopted in a serial type printing apparatus and have a short printing width at a relatively low cost. For example, Japanese Patent Publication for Opposition No. 3-5992 discloses a full-multistructure in which inkjet printheads are staggered above and below one common plate.
As advantages of using the connecting head, the manufacturing cost can be reduced by increasing the manufacturing yield, and the maximum printing width of the printhead can be relatively easily changed in accordance with the number of short-printing-width printhead chips to be arrayed.
However, the conventional “connecting head” readily degrades the print image quality at the connected portion between chips owing to the structure of the head. More specifically, the chip array shifts, the nozzle pitch of a nozzle adjacent to the connected portion changes from that at the remaining portions, and a stripe appears on a printed image.
By arraying a plurality of chips used for a short-printing-width printhead having an arbitrary number of nozzles, printheads having various printing widths can be easily configured by changing the number of chips to be arrayed. However, it is difficult to configure a printhead at a width equal to a printing width (generally a regular size) necessary for printing on a print medium. As a general arrangement, therefore, the number of chips is often increased to design the printing width of the printhead to be larger than the maximum width of a print medium. This means that many nozzles and printing elements are kept unused.
There have conventionally been proposed various solutions for the problems of the “connecting head”.
To eliminate a stripe generated at the connected portion between chips, there is proposed a method of increasing the physical machining accuracy of a printhead (e.g., an arraying method of increasing the chip array precision at the connected portion, or a method of decreasing the shift of the nozzle pitch by an alignment apparatus).
There is also proposed a method of not arraying nozzles corresponding to a connected portion so that nozzles at the ends of chips become adjacent to each other, but arraying chips so that nozzles at the ends of the chips overlap each other, and discharging ink from the overlapping nozzles so as to make a stripe inconspicuous.
Further, there is devised a method of changing the discharge amount of ink droplets discharged from nozzles at a connected portion so that printing nonuniformity at the connected portion does not stand out.
As for nozzles and printing elements which are kept unused owing to the difference between the maximum width of a print medium and the printable width of a printhead configured by arraying a plurality of chips each having an arbitrary number of nozzles, unused printing elements are not connected in the circuit configuration, and corresponding nozzles are defined as undischargeable nozzles and are not used. As a method of preventing image degradation caused by connecting chips, even unused nozzles are partially used as dischargeable nozzles in the circuit configuration when printheads are arranged in a printing apparatus so as to change the connected portion between respective colors.
Even with these measures, image degradation at the connected portion between chips cannot be completely prevented in the entire area of a printed image when images are printed on print media of various sizes by using the “connecting head”.